Path: utzoo!attcan!uunet!lll-winken!ames!mailrus!ncar!ico!rcd
From: rcd@ico.ISC.COM (Dick Dunn)
Newsgroups: comp.lang.postscript
Subject: Re: A PostScript Debugger
Summary: syntax isn't the hard part
Message-ID: <13958@ico.ISC.COM>
Date: 26 Jan 89 04:44:56 GMT
References: <6967@pucc.Princeton.EDU>
Distribution: na
Organization: Interactive Systems Corp, Boulder, CO
Lines: 38

In article <6967@pucc.Princeton.EDU>, JEFF@pucc.Princeton.EDU (Jeffrey Perry) writes:
> Does anyone know of a commercially-available program (or possibly
> a piece of shareware) that debugs Adobe PostScript syntax?  What
> I need is something to weed out my mistakes BEFORE I send a long
> file to the Laserwriter...

It wouldn't be hard to write a PostScript syntax checker--perhaps an
afternoon of C coding--but it wouldn't be very useful.  Because of the
postfix notation, the syntax is trivial, amounting to checking for tokens
being well-formed and brackets/braces matched.  In fact, I suspect that's
not the total of what you're after.  If you're looking to catch undefined
variables and procedures, mismatched types, etc., it's quite a different
matter.

In a typed, compiled language like C, this is the stuff we normally call
the "static semantics"--meaning semantic correctness which can be checked
statically.  The catch is that in PostScript it isn't static.  Names are
bound to objects dynamically, and there's nothing incorrect about having
the same name bound to a string, an integer, and a procedure in the same
dictionary at different points in execution.  (It may be bad style, but
it's not wrong.)  Names can even be manufactured by computation--a concept
which isn't meaningful in a language like C.

So how would you go about checking for the things that usually bite you (or
at least they usually bite me!:-) in developing new PostScript?  If you
explore the idea, you'll find that it eventually grows to the problem of
re-implementing a fair part of the PostScript interpreter!

What you might do, though (just an untested idea) is to build a prologue
which replaces some of the expensive operators (like show) plus the ones
that actually print (showpage, copypage) with innocuous ones.  Glue this on
to the front of your file and toss it at the printer; that will force it
through interpretation of the program, but faster than if you imaged and
printed everything.  (After all, the printer DOES contain a PostScript
interpreter.)
-- 
Dick Dunn      UUCP: {ncar,nbires}!ico!rcd           (303)449-2870
   ...A friend of the devil is a friend of mine.